Fix metadata name when ep.context_file_path is not provided

[javier-intel]

Description

When ep.context_file_path is not provided the model metadata name was missing the "_ctx" postfixed to the model name.

Motivation and Context

Conform to context model documentation

[Copilot]

Pull Request Overview

This PR updates the logic that generates the metadata filename so that when no context file path is provided, the model metadata name correctly includes the _ctx prefix before _metadata.bin.

• Introduce a name_append variable to hold the suffix.
• When session_context_.so_context_file_path is empty, set suffix to "_ctx_metadata.bin" instead of "_metadata.bin".

Comments suppressed due to low confidence (2)

onnxruntime/core/providers/openvino/openvino_execution_provider.cc:193

• [nitpick] Consider renaming name_append to something more descriptive like metadata_suffix to clarify its purpose.

      std::string name_append{"_metadata.bin"};

onnxruntime/core/providers/openvino/openvino_execution_provider.cc:198

• Add unit tests for both branches (with and without so_context_file_path) to verify that the metadata filename ends with _metadata.bin and _ctx_metadata.bin respectively.

      auto metadata_filename = metadata_file_path.stem().string() + name_append;

[MayureshV1]

LGTM ! Fixes the CreateSession Scenario where the metadata_bin file was not utilzing the ctx model name.